The present invention relates, in general, to microwave oscillator arrays, and more particularly to a quasi-optical, solid-state antenna array wherein individual oscillator elements operate at different, equally spaced frequencies coupled in such a way as to produce either a train of high-power pulses or a beam scanning repetitively through space above the array.
The generation of high power, millimeter-wave radio frequency (RF) pulses for use in communications systems has been limited, in the past, by the fact that high-current, tube-type devices not only require high voltages (in the range of 10,000 volts), but are difficult and expensive to manufacture. Furthermore, such devices are operated at a constant power level, with pulses obtained by switching them on and off. Such operation is not only inefficient, but results in undesirable switching transients. All of these factors, together with the general lack of reliability for such devices, make them unsatisfactory. Semiconductor devices have also been not been satisfactory, for in order to achieve the requisite high frequencies, they must be small in size, yet the smaller they are, the less power they can handle. Thus, a tube-type device, even with all its other shortcomings, can produce up to one kilowatt of power at 60 GHz, while solid state devices can only expect to produce up to 1 Watt at that frequency.
One way to solve the problem of producing sufficient power with semiconductor devices is to use many power sources in parallel, and in this regard, semiconductor technology is of great benefit, for it is relatively easy to duplicate multiple semiconductor devices and to connect them in parallel. A lot of work has been done in this regard, but such work has been directed primarily to operating parallel devices all at the same frequency, as described, for example, in U.S. Pat. No. 4,742,314. Such arrangements utilize a resonator structure where reflectors on opposite sides of high frequency oscillators produce a resonant cavity with the output signal leaking through one of the reflectors. Such devices are, however, limited in power intensity and directionality, and these problems are a serious deterrent to their use. Thus, although millimeter-wave systems are of interest because of their ability to operate with smaller antennas, their wider bandwidths and better resolution for imaging than existing microwave systems, nevertheless the exploitation of the millimeter-wave spectrum has been hindered by the lack of a compact, reliable, high power solid state source at these wavelengths. As noted above, high-power vacuum tube devices are available, but their large size, weight and high voltage requirements often preclude their use. The inherently small size of solid state devices makes them much more desirable, but in order to compete with vacuum tubes, solid state sources must use large numbers of devices; however, a typical solid state system would require 200 or more devices to match the output power of a single tube device.
Until recently, integrating a large number of devices had proved to be an extremely difficult task. However, a planar, quasi-optical approach to alleviate this problem is suggested by D. B. Rutledge et al in "Quasi-Optical Power Combining Arrays", invited paper, 1990, IEEE MTT-S International Microwave Symposium Digest (Dallas) May 1990. This approach uses arrays of oscillators with integrated antenna structures. Such a system forms a classical antenna array in which the power-combining is accomplished in free space so that high combining efficiencies are possible. This approach can accommodate more devices with fewer problems than the conventional methods of power combining. To date, however, work in this area has concentrated on synchronizing all elements of the array to the same frequency with identical phases. This produces a constant, coherent (CW) output signal. However, many applications require a pulsed source of brief, high-energy bursts of radiation, rather than a constant signal.